Techniques described in the following patent Literatures 1 to 3 are known as mechanisms for suppressing sidelobe levels in directional characteristics of an antenna. The Patent Literature 1 discloses a configuration in which a metal wall and a wave absorber are vertically provided on a substrate and around a quadrangular antenna element. In addition, the patent Literature 2 discloses an antenna in which a plurality of waveguide slot antennas extending in the first axis direction are arranged in the second axis direction perpendicular to the first axis direction. In this construction, a metal plate is projected in the third axis direction perpendicular to the first axis and the second axis and between the adjacent waveguide slot antennas. In addition, the patent Literature 3 discloses a structure in which a metal cover is projected in the radiation direction of electromagnetic waves and around a patch antenna. In the conventional arts, a metal plate, which projects in the radiation direction of electromagnetic waves, is provided on the above substrate to control directivity.
In addition, as a method of controlling antenna directivity in a radio wave radar, a technique is known in which a guide configured by a metal wall or the like is provided beside a radiation element to reduce a sidelobe level (e.g. refer to Patent Literature 2).